I really appreciate this recent post. It has answered a bunch of questions for me. If I may, I have one question left that I am sure you can help with. […] I am confused about how to setup the top rail with respect to BUIS and an optic system. Just to be clear, I have no idea what I really want as far as optics but I know I do want to be able to use both the optics and iron sights.

In your example, you have the fixed front sight (which I agree with, it does look proper and I don’t think it will interfere with the holo if both eyes are open). But how would you ever use the rear sight if you had to? Is it supposed to be setup so you can peer through the red dot and use the rear and front iron sights? Or, do you have to rip off the red dot to utilize the BUIS’s?

What you’re looking for are called “co-witnessed” sights, and they’re very easy to set up. Let me walk you through it…

Holographic or red dot sights are fantastic for quickly putting rounds on target. They’re nearly idiot proof aiming devices that work great especially in competition or LEO/Mil applications, or even just for shooting stuff at the range. The problem is that red dots have the ability to fail, and they usually do at the most inconvenient moments. Like, the middle of a firefight.

The solution is to mount a set of iron sights on your gun in addition to the red dot. Iron sights don’t run out of batteries and are a lot harder to damage, so chances are if the gun is still functional the iron sights will work. Especially with the more modern “flip-up” style iron sights, it’s very easy to mount a set of these puppies, zero them and then ignore them until you need them.

The most popular method of mounting and zeroing iron sights is by “co-witnessing” them with the red dot.

A “co-witnessed” set of sights means that when you’ve got your iron sights up and your red dot on, if you properly align your iron sights then the point of aim of the red dot will line up exactly with the point of aim of the iron sights. For an AR-15, that means the red dot will appear on the tip of the front sight when you have your iron sights up.

The idea is that if you align your red dot and your iron sights you have a backup aiming system on the gun. This is useful not only if your red dot goes down during a firefight, but also (since your iron sights should stay on the gun once zeroed) if you move your optic or change it out you can quickly re-zero it using the iron sights without firing a single round.

The first thing you need to do is make sure that you will be able to see your sights through the optic. With standard A4 upper receivers the front sight on the gas block and a standard rear sight (such as the Magpul MBUIS) mounted on the flattop rail will line up precisely through the middle of most optics such as an EOTech (shown here) or Aimpoint with a standard height riser. Even if you have a full length rail and no front sight on your gas block, the standard flip-up sights you can buy will typically work with the standard height optics.

Usually this will just work, as the manufacturers design their optics with this trick in mind, but if the sights or the optic is too low you may need a riser. A riser is a small platform that attaches to your rail and raises the device slightly. They come in different heights, so make sure to get the one that will lift things up just enough to get it all lined up.

Where you start to get into trouble is when you add a scope onto the gun, as the magnification combined with the lower height on the gun makes co-witnessing almost impossible. For that reason I use offset iron sights on my scoped AR-15 rifle instead.

Once you have your irons on your gun and they’re properly in line with the optic the next step is to zero the iron sights. I always like to zero the irons first as I use them as my reference point when mounting optics, so knowing that they’re precisely on target is essential. Make sure you put a good number of rounds downrange to confirm the zero.

Now you can turn your optic on. With your iron sights up, adjust the optic so that the point of aim of your red dot (or whatever) matches up with the point of aim of your iron sights. Make sure to actually aim down your sights while doing this as if you were aiming at a target. Once that’s all settled fire a few rounds using only the dot to confirm the zero.

Congratulations! You now have a red dot with co-witnessed iron sights!

At this point, I usually fold away the sights and never touch them unless I’m swapping optics. This allows the red dot to operate without any of the iron sights getting in the way, but they’re still attached and able to be quickly flipped back up if needed. But there are moments when the irons can be handy.

For example, if you need to make a precise shot on a target you may want to consider flipping open that rear sight. As good as your red dot is, there’s still a chance for some parallax to screw up your shot. By using the rear sight you can make sure your head is properly aligned and squeeze out just a hair more accuracy from your dot, as you’re now using an extra reference point to aim your gun. It may be purely psychological, but I find that it works for me (see the above video).

And that’s the benefit of co-witnessed sights and how to set them up on your gun.

About Nick Leghorn

Nick Leghorn is a gun nerd living and working in San Antonio, Texas. In his free time, he's a competition shooter (USPSA, 3-gun and NRA High Power), private pilot, and enjoys mixing statistics and science with firearms. Now on sale: Getting Started with Firearms by yours truly!

28 Responses to Ask Foghorn: How and Why to Co-Witness your Sights

It is best to learn with irons before red dots as iron sights require more consistent form than red dots. Consistent check weld and eye placement are key with irons. Best to learn the form for shooting irons and maintain it for use with the red dot. Irons are meant to be shot “nose the the charging handle” which is closer than many people shot with red dots. I find that people that learn with red dots before irons, don’t learn this and then tend to have trouble with their irons.

I do a 1/3 co-witness of my Aimpoint red dots which places the red dot sight slighter above the plane of the irons which lowers the irons in my field of view. This is accomplished by using a slightly taller mount for the red dot. This approach yields a cleaner, easier to use, sight picture in my opinion.

Personally, I leave my folding sights unfolded all the time. If my red dot goes down I can immediately transition my eyes to the irons without having to waste the time to flip up both sights.

Since is it possible for impact damage, snow, fog etc. to impair the ability to see through the red dot, I use quick detach mounts for all my red dots. If I can’t see through the red dot sight for any reason, it is easy to remove.

“It is essential that the shooter knows how to shoot with iron sights!”
That cannot be said enough.

“I do a 1/3 co-witness”
Same here, but that is just my preference. Try both and see what you like.

“I leave my folding sights unfolded all the time.”
I use front folding sights for a reason, when folded they don’t snag near as much. When crawling through brush after hogs I prefer mine folded, but for tactical use they are always up unless using a magnified scope.

“I use quick detach mounts for all my red dots”
All my scopes for me, for the same reasons.

I would recommend using the term “back up sights” not “iron sights”. It is more accurate exspecially since many of the folding back up sights, including the popular MagPul MBUS, are not iron. Other than that a great article with good advice.

A.R.M.S. and some of the other smaller companies. I’ve heard talk of some from the other big co.s but don’t know if there is any truth to those rumors. Due to cost and their loyal following the MBUSs seem to be becoming the norm.

I read articles like this for ideas and information for when I move on to something more complicated than just the “irons”. In my case they are actually folding aluminum front and rear (QDS from YHM). I passed over the MBUS parts because I knew up front I would be using the “irons” as primary sights for some time.

Great article. This helps since I have a 512 Eotech and I also run my Bushmaster ACR with Magpul backup sights. My question is; what method do you suggest to zero in the backup sights? Sould I zero in at 25/100 or I’ve also heard military likes to zero sights at 300m. So what would you recommend? Thanks

Hello there, on the “co-witness” topic can you give me a few suggestions on a red dot or holographic optic that would work with or should i say allign with my magpul gen 2 sights, I know about the $600 and up set ups but some descent optics in the $100 to $250 range? My rifle is a flattop M4 with a Spector length rail and low pro gas block…… Thanks

Does it make sense to use a metal backup sight on a gas block rail? I guess the heat from the gas block could cause issues with non-iron material. I am planning to purchase backup’s for my rifle too. A knowledgeable guy at Cabela’s mentioned the importance of metal sights upfront on my gas block rail.

Great article as I am going to build an AR this fall, and I’m looking into optics.

Question concerning co-witnessing iron sights and red dots. One of the issues I have in my climate is snow, sleet and rain. My deer rifle has see through scope mounts so I can switch to iron sights when bad weather won’t allow me to see through the scope.

Would it make sense to pair a red dot with offset iron sights in bad weather climates for this reason, or is that not an issue? I’ve never shot with a co-witnessed sighting system.

Loved your reasoning between the setups in your red dot and scoped rifles.

Hi, saw you website and enjoy the post. New to the AR family and I have a few questions… I currently own a Core Scout AR-15. This unit came with a fixed iron sight and no rear. I purchased a rear “flip-up”
iron sight and it works reasonably. My problem is my eyesight is an issue with iron sights, whereas I have diminished vision with moderate blur especially at around 60 yards. I have shot several AR’s with red dots only with no fixed front and I fair much better. I’d like to add a red-dot to this unit and keep the fixed front sight. Any recommendations using a co-witnessed reasonable cost red-dot that will do the job? Any help and/or recommendations appreciated….

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